1. Field of the Invention
The present invention relates to a novel Ni complex able to remove free radicals, its derivatives, its producing method, and compositions containing the Ni complex or its derivative. More particularly, the invented Ni complex and its derivatives can mimic the active site of the Ni-containing superoxide dismutase (NiSOD).
2. Description of the Related Art
Free radicals, such as superoxide radical, hydroxyl radical, methyl radical, correlate with senescence and most diseases of human bodies. Free radicals are very active and participate in chain reactions inside cells, overoxidizing the lipid of cellular biomembranes and destroying the structures and functions thereof. Free radicals may also denature or crosslink proteins, disabling enzymes and hormones, and degrading immunity, reflex and mobility of human bodies. Further, free radicals may damage structures of nucleic acids, cause disorders of metabolism, and finally bring about diseases of organisms.
Although free radicals may result in various harms to human bodies, human bodies possess systems to remove or inhibit free radicals. Some of the abovementioned systems are implemented by antioxidative enzymes, which function as antioxidants in the systems. Superoxide dismutases (SODs) are the main antioxidative enzymes, which can eliminate superoxide radicals and thus play an important role in defending against the toxicity of oxygen, decelerating senescence, and preventing from senile diseases.
Superoxide dismutases (SODs) are metalloenzymes and can be categorized into three types: the copper and zinc-containing superoxide dismutase (CuZnSOD), the manganese-containing superoxide dismutase (MnSOD) or the iron-containing superoxide dismutase (FeSOD), and the nickel-containing superoxide dismutase (NiSOD). SODs can catalyze the dismutation of superoxide radicals and convert them into oxygen and hydrogen peroxide.
Recently, it was found that the Ni-containing superoxide dismutase isolated from streptomyces and marine cyanobacteria can catalyze the dismutation of O2− into O2 and H2O2 through a cycle of nickel(II) and nickel(III) oxidation states (H.-D. Youn, et al., Arch. Biochem. Biophys. 1996, 334, 341-348; B. Palenik, et al., Nature 2003, 424, 1037-1042). In these papers, it is reported that the active site of the reduced NiSOD has a nickel(II) ion and the active site of the oxidized NiSOD has a nickel(III) ion. The coordinations of the reduced NiSOD and the oxidized NiSOD are shown as follows:

So far have been synthesized several model compounds with a N2S2 square planar geometry for the active site of NiSOD (J. J. Smee, et al., Inorg. Chem. 2001, 40, 3601-3605). However, only peptide-supported model compounds have been proved to have the NiSOD-like catalytic ability (J. Shearer, L. M. Long, Inorg. Chem. 2006, 45, 2358-2360). No synthetic model has shown the function yet.